According to mass-energy equivalence, every gram of water contains ~2.15*10^13 calories.

So where can I sign up for having my stomach replaced with a matter-antimater reactor or whatever it is this gadget that actually gives me access to this energy?

I think you're missing an algebraic sign there.

I assume you are talking on the physics e=mc^2 type level rather than an energy store that can be metabolized by your body.